Simplify the following expression: $ n = \dfrac{x - 3}{9} + \dfrac{-4}{7} $
In order to add expressions, they must have a common denominator. Multiply the first expression by $\dfrac{7}{7}$ $ \dfrac{x - 3}{9} \times \dfrac{7}{7} = \dfrac{7x - 21}{63} $ Multiply the second expression by $\dfrac{9}{9}$ $ \dfrac{-4}{7} \times \dfrac{9}{9} = \dfrac{-36}{63} $ Therefore $ n = \dfrac{7x - 21}{63} + \dfrac{-36}{63} $ Now the expressions have the same denominator we can simply add the numerators: $n = \dfrac{7x - 21 - 36}{63} $ $n = \dfrac{7x - 57}{63}$